The optical navigation sensors used in optical mice detect the changing patterns of light that are reflected from microscopic textural features of the surface over which they navigate. The responsivity of a navigation sensor plays an important part in how well a mouse tracks the features of a surface. This is especially so in the case of surfaces having few surface features, such as glass surfaces.
Typically, the responsivity of an optical navigation sensor is adjusted by changing the doping level of a phototransistor's emitter, or by changing the width of a phototransistor's base, to increase the phototransistor's beta value (i.e., current gain). However, there is a limitation to increasing the beta value of a phototransistor, as its doping level or width cannot be changed beyond certain “threshold” values, beyond which the fundamental transistor action is adversely affected. For instance, the base width could not be narrowed too greatly such that it affects the matching among the pixels. In addition, the gain improvement rendered by this method is limited by shot noise. In other words, even though a desired current gain may be attainable, the signal-to-noise ratio (SNR) may remain the same or become worse in that the noise floor may increase inadvertently with the increase of the beta/current gain.